This is one of those posts that will help one person out there who is googling around for a power adapter for a Western Digital MyBook Essential (or other MyBook model). (Well, rereading it, it is somewhat useful for learning to match power adapters to electronic devices in general.)
The Western Digital MyBook Essential is a USB hard drive which comes in 1TB, 2TB, and 3TB sizes and is very economical for the space you get, and has slightly more expensive models that support built-in full disk encryption. Unfortunately it is useless if you do not have a power adapter (which it comes with but is easy to lose), because USB does not supply sufficient power to drive the 3.5" desktop-sized hard drive it houses. After some research it looks like the power adapter is 12v center positive. I have a multi-volt power adapter that works great (solid constant power even when powered from a car inverter.... don't ask) but it is only 300ma. The higher powered multivolt adapters I've purchased from Radioshack and Fry's tend to have dirty power when driven from a car inverter or simply don't work for some reason - these have all ended up in the trash. The 300ma power adapter is not enough to power the MyBook USB hard drive, which comes with a 1.5 amp power supply, so it was back to the drawing board. I remembered from robot labs that Bioloid AX-12 etc. actuators use 12v and have huge amperage - 5 amps (60 watts total!). Looking at the ax-12 adapter, it is indeed center positive (also called "tip positive") so I tried the plug to see if it fits and... viola', MyBook works like a charm.
Although trying reversed polarity seems like it would break electronics in my experience the electronics do not break on reversed polarity but simply don't turn on (I tried this while playing with some electronics I never expected to work, so the risk was low, and they eventually did work so I learned reversed polarity didn't break them). This doesn't apply to anything we did above because we checked and matched polarity properly - I digress.
Now to find the USB 3.0->USB 2.0 adapter, ugh!